The hydrated structure of ammonia molecule in aqueous solution was theoretically optimized as an ammonia–water molecule pair (H3N⋯H2O) by the free energy gradient (FEG) method [J. Chem. Phys. 113, 3516 (2000)]. The interaction between the pair and a solvent water molecule (TIP3P) [J. Chem. Phys. 79, 926 (1983)] was described by a hybrid quantum mechanical and molecular mechanical method combined with a semiempirical molecular orbital method at the PM3 level of theory. It is concluded that the present FEG method works quite well in spite of a simple steepest descent optimization scheme equipped with the adaptive displacement vector. The free energy stabilization was estimated −0.3 kcal/mol from the free energy for the same structure as that of the cluster in the gas phase. The optimized structure was found to be almost the same as that in the gas phase except for a longer OH bond length of the water molecule. However, its realization in aqueous solution is accomplished by virtue of the fulfillment of both “zero gradient” and “force balance” conditions. Finally, we discuss also the effect of microscopic “solvation entropy,” compared with the result by the conductorlike screening model method.

For the ionization process of ammonia in aqueous solution, the transition-state (TS) structure was fully optimized for the first time on the free-energy surface (FES) by applying the free-energy gradient (FEG) method combined with a hybrid quantum mechanical and molecular mechanical molecular dynamics (QM/MM-MD) method. In aqueous solution, the ionization process was found to proceed by way of a clear TS (R(N1-H5) = 1.512 A), which does not exist in the gas phase. The free-energy (FE) of activation for ionization obtained was 14.7 kcal/mol, within the classical approximation, via the QM/MM-MD FEG method, and is found to be in good agreement with 9.57 kcal/mol estimated from the TS theory using the experimental value of the rate constant. Apart from the dynamic correction, it is indicated that the theoretical value would be improved to be 10.28 kcal/mol if the electronic-state calculation could be executed at the B3LYP/6-31G(d) level of theory.

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